Rotation. It sounds pretty easy and in fact it is pretty easy to rotate when you are standing straight up and down. However when you add the bend at the hips so that you can reach the golf ball and all of the sudden it becomes a little more complicated.
A common issue that impacts a number of golfers is they sway as opposed to rotating in the back swing. When you sway, there is excessive lateral motion and it really hinders you from having a stable spine. Swaying moves the angles of all the key joints in your body so that the trunk (spine angle) isn't moving in a stable rotation. Generally when you sway on the backswing you will have to compensate in the downswing to get your spine back to the proper position. Added complexity of how the core body moves adds complexity to how you have to manage getting the club onto the ball in a repeatable way.
When the spine is more stable you can rotate faster and faster and keep the club moving in a simple motion. While there certainly is an overall lateral shift towards the target during the swing, most great golfers don't sway away from the target in the backswing,
Look at the setup and top of the backswing photos of Adam Scott and Tiger Woods. You won't generally see PGA players move their trail leg away from the target in the backswing (sway).
When you practice with the Trackman, take the time to draw a line on your trail leg at setup and track the motion of the trail leg through the backswing. If you materially cross that line you are starting to sway as opposed to rotate in your backswing.
Rob Stanger has some pointers on how to rotate vs sway in your swing motion.